This invention relates to the field of ink drop printing. Systems for performing ink drop printing have undergone substantial development in recent years and are utilized for a variety of applications. Typical applications are for word processing, as shown in Hill et al U.S. Pat. No. 3,964,591, forms printing, as shown in MacIlvaine U.S. Pat. No. 3,911,818, bar code printing, as shown in Duffield U.S. Pat. No. 3,787,881, document addressing, as shown in Bok et al U.S. Pat. No. 4,283,731, facsimile reproduction, as shown in Van Brimer et al Nos. 3,588,906, document copying, as shown in Van Hook 4,009,332, and duplicating, as shown in Paranjpe et al 4,259,696. Techniques for drop generation and control include drop-on-demand systems as shown in Stemme et al 3,747,120, non-stimulated jets as shown in Hertz 3,416,153, stimulated and variably deflected jets as shown in Lewis et al 3,298,030 and stimulated and binarily switched jets as shown in Sweet et al 3,373,437. The art has now become quite well developed, and there are many variations of each of the above mentioned ink drop printers.
Generally speaking, prior art ink drop printers have been either relatively slow in their operation or relatively limited in the resolution of printed output. The high resolution printers have generally utilized a single drop generator or a group of relatively few drop generators which necessarily required a relatively long time to complete a sizable printing job. Ink drop printers of the type disclosed in the above mentioned Sweet et al patent are able to operate at very high speeds, but their resolution has been limited due to the difficulty of placing a great many drop generators close enough together to meet the operating constraints of that particular type of printer.
One solution to the problem of obtaining high speed and high resolution has been to provide a plurality of parallel staggered and laterally extending print bars as taught in Taylor et al RE No. 28,219. However, in order to meet the resolution requirements associated with document duplicating, the system of Taylor et al requires a fairly large number of print heads, and this in turn creates manufacturing costs which are non-competitive with more conventional duplicating systems.
One approach to a cost competitive ink drop duplicator in shown in Paranjpe et al U.S. Pat. No. 4,259,696. In the Paranjpe patent a single printing head is oscillated back and forth across a plurality of printing positions. Copy sheets are transported under the printing head as it is oscillated back and forth. Switching control information for the printing jets is generated by scanning an original document with a photodetecting array. The photodetecting elements comprising the array are arranged in a manner similar to the arrangement of jets in the ink drop printing head. Scanning of the document proceeds in a manner similar to the pattern of scanning of the print head over the copy sheet. This system requires that an original document be scanned one time for each copy printed by the print head, so that the scanning device must be moved at very high scanning rates and very high levels of illumination must be employed. The high speed scanning and the mechanical oscillation of the jets also create severe design problems.